1. Field of the Invention
The present invention relates generally to integrated circuitry that is compensated for process and operating condition variations. More particularly, the present invention relates to output buffers and amplifiers that are fabricated at least in part on an integrated circuit chip (IC Chip) and that are immune to process and operating condition variations encountered on the IC Chip.
2. Description of the Related Art
An IC Chip often is used to process signals from other circuitry. To communicate with such circuitry, the IC Chip includes a plurality of input buffers and output buffers. The input buffers allow the IC Chip to receive signals for processing without being damaged by voltage or power spikes in the signals. The received signals are then processed by the IC Chip, and in some instances an output signal is prepared for circuitry off the IC Chip The output buffers allow the IC Chip to supply such output signals with sufficient power, and protect the IC Chip from voltage and power spikes from other circuitry.
One type of conventional output buffer included on IC Chips comprises a pre-driver and an n-channel field effect transistor (NFET). The pre-driver has an input that receives a control signal from other circuitry on the IC Chip. The control signal indicates whether or not the output buffer will supply an output signal. The pre-driver also has an output that supplies a buffered control signal in response to the control signal received. This buffered control signal generally has greater signal strength than the control signal.
The NFET has a gate, source, and drain. The gate is coupled to the output of the pre-driver, and hence receives the buffered control signal. The source is coupled to a ground voltage supply. The ground voltage supply supplies electrical power to the NFET. The drain serves as an output port of the output buffer, and thus asserts output signals for circuitry off board the IC Chip.
In operation, the pre-driver turns ON the NFET to assert a LOW output signal at the drain. The low output signal is at the ground voltage level. To assert no output signal, the pre-driver turns OFF the NFET. Those skilled in the art will recognize that this conventional output buffer can be modified to assert HIGH output signals at a high voltage level, or to output signals both high and low output signals.
Conventional output buffers have several limitations. First, conventional output buffers of the same design often operate at different speeds due to variations in the manufacturing process used to fabricate the IC Chips. Second, transistors included in conventional output buffers usually operate more slowly as the IC Chip heats up. Because high speed IC Chips operate at very high temperatures, this limitation significantly reduces the rate at which signals can be output from such output buffers.
A third problem with conventional output buffers involves instability in the voltage level of output signals as the slew-rate of the output buffers is increased. The slew-rate of an output buffer (or transistor) is defined as the maximum rate of change in the voltage level of a signal output therefrom. For example, in an output buffer having a slew-rate of 0.3 Volts per nanosecond, approximately 10 nanoseconds are required to change the output signal from 0 Volts to 3 Volts.
The slew-rate can be increased to an extent to increase the speed of the output buffer. Unfortunately, as the slew-rate is increased in conventional output buffers, the value of the output signal can become unstable. At high slew-rates, the output signal will typically change to approximately the proper value quickly, and then return at least partially to its prior value. The value of the output signal often bounces back and forth until finally stabilizing around the proper value. Circuitry coupled to the output port of the output buffer will thus receive a bouncing output signal. Such circuitry often reads the wrong value from this output signal because it is bouncing. This problem with conventional output buffers prevents an IC Chip from communicating properly with other circuitry, which in turn leads to erroneous results.
The slew-rate of a conventional output buffer fabricated on an IC Chip is typically heavily dependent on variations in the manufacturing process used for fabricating the IC Chip, and on the particular operating conditions encountered in the IC Chip. To account for such variations, conventional output buffers typically are designed to operate well below an optimal slew-rate. Furthermore, the slew-rate typically will vary to a substantially degree during operation of the IC Chip. For example, as the IC Chip heats up, the slew-rate usually drops significantly. Consequently, it is very difficult to maintain the slew-rate of the conventional output buffer within the input tolerance of circuitry coupled to the output buffer.
There is thus a continuing need in the area of semi-conductor electronics for an improved output buffer wherein both the speed and the slew-rate of the output buffer can be maintained within narrow tolerances. Such an improved output buffer preferably should not encounter the instability of conventional output buffers. Further, such an improved output buffer preferably should be immune to variations in performance despite any variations in the manufacturing process or operating conditions of the output buffer.